The objective is to investigate the role of prolactin and growth hormone in relation to genetic, viral, chemical and nutritional factors in the development of mammary tumors in mice. The dynamics of prolactin secretion (induced with prehenazine or breast nursing) and metabolic clearance rates (MCR) will be compared after foster-nursing (MTV introduction) between mice of high and low incidence strains, after administration of chemical carcinogens, and after the induction of obesity. Prolactin and growth hormone will be measured by specific and homologous radioimmunoassay for the two hormones. The modulation of prolactin and growth hormone receptor activities in the mammary gland by genetic, viral, hormonal and nutritional variables will be investigated. Mammary receptor activites will be measured by in vitro incubation of mammary plasma membrane fractions with mouse prolactin and growth hormone iodinated with lactoperoxidase. The influence on mammary tumor growth of immunization against mouse prolactin and growth hormone by administration purified anti-mouse prolacting and anti-growth hormone gamma globulins will be studied.